As a method for forming an L2VPN through the Internet, a tunneling method is one of well-known ones. According to this tunneling method, an ether frame flowing in a LAN connected to a sender's VPN device is caught and encapsulated with the IP, then sent to its receiver's VPN device. Receiving the IP packet that is an encapsulated ether frame, the receiver's VPN device takes out the ether frame from the received IP packet, then sends the ether frame to a LAN connected to the receiver's VPN device. The L2VPN is formed in such way.
Communications through the Internet have always been confronted with such problems as wiretapping and illegal altering by third parties in communication channels. This is why data that must be sent/received safely has been required to be encrypted. The IPsec (IP Security Protocol) is one of the generally known methods for protecting data in IP packets from such wiretapping and illegal altering (for example, refer to the non-patent document 1). The IPsec encrypts/decrypts IP packets according to a security association in which the necessary encrypting method and key are defined beforehand. The security association is established dynamically through an IKE (Internet Key Exchange) operation or it is set manually beforehand.
The multicasting method is known well as a technique for sending IP packets having the same content from one terminal to a plurality of terminals. In case of the unicasting communication method, if there are a plurality of terminals of receivers, the sender's terminal is required to copy each IP packet to be sent as many as the number of the terminals of the receivers. In case of the multicasting communication method, the sender's terminal is required to send only one IP packet and a proper multicast router existing in the communication channel copies the IP packet. The sender's terminal can thus reduce both of the processing load and the traffic, thereby the terminal comes to use the network efficiently.
[Non-patent document 1] IETF, RFC2401